2 Abstract Liquid argon Time Projection Chambers (TPCs) are dense, fine grained, fully homogeneous particle detectors with excellent calorimetric and three-dimensional imaging capabilities. The fact that this technology allows to build massive detectors, from the ton to the many kton scale, depending on the application, makes it attractive for neutrino physics, proton decay and direct Dark Matter searches. The amplification of the ionization charge produced in liquid argon improves the performance of a liquid argon TPC, giving a larger signal to noise ratio and, as a consequence, a lower energy threshold. This amplification can be achieved extracting the ionization electrons from the liquid into the vapor phase, and multiplying them using a Large Electron Multiplier (LEM). This new kind of Time Projection Chamber is called double phase argon LEM-TPC and represents the main topic of this work. The goal of the thesis is to understand if the double phase argon LEM-TPC is a valid concept, to estimate its performance and to identify possible problems. The LEM is a metal cladded epoxy plate (PCB), with a thickness of about a millimeter and with mechanically drilled holes. Typically, these holes have diameters of less than a millimeter, and there are order of 100 holes per cm 2 of LEM. The ionization charge (electrons) drifts in liquid argon from the interaction point towards the liquid-vapor interface under the action of a uniform electric field. The electrons are then extracted into the vapor by means of an electric field applied across the liquid surface using two grids at different potentials (one in liquid and one in gas). The electrons are focalized into the holes of the LEM, where the electric field is high enough to produce charge multiplication via Townsend avalanche. Finally, the electrons induce signals on a two views anode, which has the electrodes segmented into strips along two orthogonal directions and, therefore, gives the position of the ionizing event projected onto the anode plane. The third coordinate is given by the drift time multiplied by the electron drift velocity. In this thesis we present the modeling, the design, the construction and the operation of a prototype of a double phase argon LEM-TPC with an active volume up to 3 L and with a LEM area of cm 2. This is the first detector of this kind to be fully operational. We describe the framework used to model and design the charge amplification and readout system, give a detailed description of the experimental setup, and analyze the data acquired in the exposure i

3 ii of the detector to cosmic ray muons. This measurement campaign has allowed to benchmark the detector performance and to fix the design parameters of the charge readout system. The maximum effective amplification obtained in stable condition was about 30, that corresponds to a signal to noise ratio for minimum ionizing particles of more than 200. The experience gained during the extensive R&D campaign on the 3 L detector allowed to design and construct a larger double phase argon LEM-TPC with an active area of cm 2, which was successfully operated in Here we report the first results of a test with cosmic ray muons.

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